High concentration NO2  generating system and method for generating high concentration NO2 using the generating system

ABSTRACT

A high concentration NO 2  gas generating system including a circulating path configured by connecting a chamber, a plasma generator, and a circulating means, wherein NO 2  is generated by circulating a gas mixture including nitrogen and oxygen in the circulating path is provided. The high concentration NO 2  gas generating system provides a high concentration NO 2  generating system and the high concentration NO 2  generating method using the generating system by which NO 2  of high concentration (approximately 500 ppm or above) required for a high level of sterilization process in such as sterilization of medical instruments can be simply and selectively obtained. In addition, since indoor air is used as an ingredient, the management of ingredients is simple and highly safe, and the high concentration of NO 2  can be simply and selectively prepared on demand.

TECHNICAL FIELD

The present invention relates to a high concentration NO₂ generatingsystem and a method for generating high concentration NO₂ using thegenerating system. More particularly, the present invention relates to ahigh concentration NO₂ generating system for obtaining NO₂ of highconcentration in a simple and selective manner by using the air as aningredient, and a method for generating a high concentration NO₂ usingthe generating system.

BACKGROUND ART

Conventionally, as sterilizing methods of medical instruments,high-pressure steam sterilization (hereinafter, simply referred to as“AC”) and ethylene oxide gas sterilization (hereinafter, simply referredto as “EOG sterilization”) have been widely used.

AC is a sterilization method in which an item to be sterilized isexposed under a high temperature at approximately 135° C., and has beenwidely used for medical instruments made of metal. However, there is adisadvantage that limitations exist in items to be sterilized sincesterilization is performed under a high temperature condition. Forexample, there is a problem that heat labile materials such as plasticscannot be sterilized by AC.

On the other hand, EOG sterilization can be used for plastics since itcan be performed at a lower temperature of 70° C. or below. However, dueto its toxicity and risk of explosion, there is a disadvantage that EOGneeds to be securely stored so as not to cause a problem associated withhygienics and safety, and sufficient care needs to be taken in handling.In addition, when EOG is supplied from a tank (cylinder) to asterilizing apparatus via a pipe, the occurrence of weight reductionneeds to be constantly monitored by measuring the weight of the cylinderfor the purpose of preventing unexpected leakage from such as the pipes.

Besides those sterilization methods, a sterilization method usinghydrogen peroxide (H₂O₂) has been used. As compared with EOG, hydrogenperoxide is simple to use and manage, and is useful from the safetyperspective. However, since hydrogen peroxide is used in the form of anaqueous solution, the permeability to detail portions such as an insideof a tube is inferior to the AC or EOG sterilization.

As an alternative method to the AC or EOG sterilization, as shown inJapanese Unexamined Patent Publication No. 240864/1988, a sterilizationmethod using high concentration ozone (O₃) has also been used in whichhigh concentration ozone is generated by providing a circulating pump ina position downstream from the ozone tank and upstream from the ozonizerand circulating ozone therethrough. In the method, an advantage existsthat the generation of ozone and the decomposition of ozone after useare simple. However, there are disadvantages that high concentrationozone is explosive and gives the substantial damage to plastics.

As a sterilization method with no risk of explosion as compared with theabove-mentioned various sterilization methods, a sterilization methodusing a nitrogen oxide gas (hereinafter, also simply referred to as“NOx”) has been proposed. In the method of Japanese Unexamined PatentPublication No. 162276/1983, for example, a gas mixture which isobtained by performing a plasma treatment to the gas mixture of oxygenand nitrogen is used for the purpose of sterilizing Escherichia colipresent on such as food surface. In the method, a gas mixture ofnitrogen oxide and ozone is prepared by performing the plasma treatmentto a gas mixture introduced from an oxygen cylinder and nitrogencylinder. The prepared gas mixture is sprayed on the surface of food tosterilize Escherichia coli present on the surface. Since thesterilization process can be performed at a moderate temperature, thereare advantages that the method can be used for various items to besterilized, and that sterilant gas does not need to be stored sincenitrogen oxide is generated on demand.

DISCLOSURE OF INVENTION

In the sterilizing apparatus of Japanese Unexamined Patent PublicationNo. 162276/1983, however, nitrogen oxide is prepared by so called“single pass”, a single plasma treatment of a gas mixture of oxygen andnitrogen. In addition, nitrogen oxide is sprayed on the surface of foodin an open space and the nitrogen oxide after the treatment is directlyreleased to the atmosphere. As a result, the concentration of sterilantgas including nitrogen oxide is, at most, an order of several ppm and isuseful to the extent of sterilizing Escherichia coli (and sterilizationis performed on Escherichia coli present only on the surface of food).Accordingly, there is a problem that the method can never be used forthe purpose of a high level of sterilization where enhanced reliabilityis desired (for example, medical instruments attached with germs; morespecifically, sterilization of a microspace such as between scissors andan inside of a tube).

The present invention is provided in view of the above-mentionedproblems. By focusing on the fact that nitrogen dioxide (hereinafter,also simply referred to as “NO₂”) exhibits a high sterilizing effectamong other sterilant gases including nitrogen oxide, an object of thepresent invention is to provide a high concentration NO₂ generatingsystem and a method for generating high concentration NO₂ using thegenerating system, by which NO₂ of high concentration (approximately 500ppm or above) required for a high level of sterilization process in suchas sterilization of medical instruments can be simply and selectivelyobtained. Another object of the present invention is to provide a highconcentration NO₂ generating system and a method for generating highconcentration NO₂ using the generating system, by which the highconcentration of NO₂ can be simply and selectively prepared on demandsince the management of ingredients is unnecessary due to the use ofindoor air as an ingredient.

The high concentration NO₂ generating system according to the presentinvention is shown in FIG. 1.

The system is configured by a chamber, NO₂ sensor, flow resistor, flowmeter, plasma generator, pressure meter, circulating pump, dry airsupply apparatus, and exhaust pump.

Preferably, the circulating means is a pressure device, and thecirculating path is configured by connecting the plasma generator to thechamber at a downstream side of the path, connecting the pressure deviceto the plasma generator at a downstream side of the path, and connectingthe chamber to the pressure device at a downstream side of the path.

Preferably, a flow resistive portion is connected between the chamberand the plasma generator.

Preferably, the circulating path further includes an NO₂ concentrationmeasuring means.

Preferably, the NO₂ concentration measuring means is disposed within thechamber, or between the chamber and the flow resistive portion.

Preferably, the circulating path further includes an inlet portion forintroducing the gas mixture, and the inlet portion includes a closuremeans and a gas drying means.

Preferably, the closure means is closed by detecting an internalpressure in the circulating path which increases by supplying the gasmixture into the circulating path under a reduced pressure.

Preferably, the flow resistive portion is an orifice.

The present invention is a method for generating high concentration NO₂,including circulating an NOx gas mixture in a circulating path formed bya chamber, a plasma generator, and a circulating means until NO₂concentration reaches 500 ppm to 100,000 ppm by using the highconcentration NO₂ gas generating system.

Preferably, ambient air is employed for the gas mixture.

Preferably, dry air with a dew point from 0 to −90° C. is used for thegas mixture.

Preferably, an internal pressure of a plasma generating portion of theplasma generator is from 20 to 90 kPa (absolute pressure).

In the case the safety is a significant concern, preferably, a pressuredifference between atmospheric pressure and an internal pressure of aninterval from the pressure device through the chamber connected to thepressure device at the downstream side of the path to the flow resistiveportion connected to the chamber at the downstream side of the path isset between approximately −1 and −50 kPa (relative pressure).

In the case the compactness of the system is a significant concern, aninternal pressure of an interval from the pressure device through thechamber connected to the pressure device at the downstream side of thepath to the flow resistive portion connected to the chamber at thedownstream side of the path is maintained to be a positive pressurerelative to atmospheric pressure.

Preferably, a flow volume of the NOx gas mixture circulating in thecirculating path is 5 LPM or above.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an explanatory view illustrating a high concentration NO₂generating system according to an Embodiment of the present invention.

FIG. 2 is an explanatory view illustrating a plasma generating portionin the high concentration NO₂ generating system according to anEmbodiment of the present invention.

FIG. 3 is an explanatory view illustrating a plasma generator in thehigh concentration NO₂ generating system according to an Embodiment ofthe present invention.

FIG. 4 is an explanatory view illustrating a circulating path used in anEmbodiment (Examples 1 to 3 and Comparative Example 4) of the presentinvention.

FIG. 5 is a graph illustrating the results of Examples 1 to 3.

FIG. 6 is an explanatory view illustrating a circulating path used in anEmbodiment (Examples 4 to 6) of the present invention.

FIG. 7 is a graph illustrating the results of Examples 4 to 6.

FIG. 8 is an explanatory view illustrating a circulating path used in anEmbodiment (Comparative Examples 1 and 2) of the present invention.

FIG. 9 is a graph illustrating the results of Comparative Examples 1 and2.

FIG. 10 is an explanatory view illustrating a circulating path used inan Embodiment (Examples 7 to 13 and Comparative Example 3) of thepresent invention.

FIG. 11 is an explanatory view illustrating a circulating path used inan Embodiment (Comparative Examples 5 to 7) of the present invention.

FIG. 12 is a graph illustrating the results of Examples 8 to 13.

FIG. 13 is a graph illustrating the results of Example 14.

BEST MODE FOR CARRYING OUT THE INVENTION

The high concentration NO₂ gas generating system according to thepresent Embodiment includes a circulating path 4 in which a chamber 1, aplasma generator 2, and a circulating apparatus 3 are connected as shownin FIG. 1. More specifically, the circulating path 4 is configured toinclude the chamber 1, a flow resistive portion 5 connected to thechamber 1 at the downstream side of the path via a pipe, the plasmagenerator 2 connected to the flow resistive portion 5 at the downstreamside of the path via a pipe, and the circulating apparatus 3 connectedto the plasma generator 2 at the downstream side of the path via a pipe.The circulating apparatus 3 is further connected to the chamber 1 at theupstream side of the path via a pipe such that a cyclic circulating path4 is formed by the chamber 1, flow resistive portion 5, plasma generator2, and circulating apparatus 3. By the operation of the circulatingapparatus 3, a gas mixture including nitrogen and oxygen circulates inthe circulating path 4 to generate NO₂.

The chamber 1 is an airtight compartment for containing a highconcentration NO₂ gas to be generated. The chamber 1 has a rectangularbox shape in the present Embodiment, however, it may have a sphericalshape or cylindrical shape. Since the chamber 1 of the presentEmbodiment forms the circulating path 4, a flow outlet, a flow inlet,and an openable and closable gas supply opening for taking out the highconcentration NO₂ gas are formed.

The chamber 1 of the present Embodiment includes an inlet portion 1 afor introducing the gas mixture therein and a gas supply portion 1 b forexhausting the gas in the chamber 1. The inlet portion 1 a includes aninlet pipe 1 c connected to the inlet opening of the chamber 1, aclosure means 1 d for opening/closing for an air flow through the inletpipe 1 c, and a gas drying means 1 e for drying the gas mixture. Thereference numeral f is a filter. The gas drying means 1 e dries the gasmixture introduced in the chamber 1 to prevent impurities from attachingto such as the plasma generator 2, and to prevent corrosion of thecomponents such as an electrode and packing by inhibiting nitrificationof NOx. In the present Embodiment, an air dryer is employed as the gasdrying means 1 e.

In the present Embodiment, an air drive valve is used as the closuremeans 1 d. With a pressure detecting means 8 (pressure sensor), the airdrive valve detects the pressure in the circulating path 4 whichincreases by supplying a gas mixture into the circulating path 4controlled under a reduced pressure. The air drive valve is controlledsuch that it is electrically driven to close when the differentialpressure relative to the ambient air is approximately from −1 to −50 kPa(relative pressure). In addition to this, an air drive valve or anelectromagnetic valve which automatically closes at a predeterminedinternal pressure may be employed as a closure means 1 d. Furthermore,the closure means 1 d may be configured by a gas drying means 1 eincluding a valve for blocking an air flow at the same time of shuttingdown operation.

It is noted that the inlet portion 1 a for introducing the gas mixturemay be provided in the circulating path besides the chamber 1, and maybe connected, for example, to the pipe at the upstream side from theplasma generator 2.

The gas mixture is a gas including nitrogen and oxygen which areingredients for generating high concentration NO₂ gas, and the air isemployed as the gas mixture in the present Embodiment. In the presentEmbodiment, a tip of the inlet pipe 1 c, therefore, is open for servingas an air inlet opening and includes an air filter. In addition to theair, a gas in which nitrogen and oxygen are composed at a ratio ofbetween 95:5 and 5:95 and which is filled in a cylinder may be employedas a gas mixture. In such a case, the tip of the inlet pipe is connectedto the cylinder.

The gas supply portion 1 b includes an exhaust pipe 1 f, a closure means1 g, and an exhaust pump 1 h. By opening/closing the closure means 1 gand driving the exhaust pump 1 h, the high concentration NO₂ gasretained in the chamber 1 including the circulating path 4 as well asimpurities and gases such as vapors can be exhausted in an exhaustingstep described later. In the case of the high concentration NO₂ gas, byconnecting the gas supply portion 1 b to a sterilizing chamber forperforming a high level of sterilization of such as medical instrumentsto exhaust the gas, a sterilization apparatus using the highconcentration NO₂ gas on demand can be formed.

In the present description, a gas including nitrogen and oxygen which issupplied from the outside to the high concentration NO₂ generatingsystem as an ingredient is referred to as a gas mixture, a gas includingNOx which is generated by circulating through the plasma generator 2 atleast once is referred to as an NOx gas mixture, and a gas which reachesa desired level of NO₂ concentration by repeating the above-describedcirculation is referred to as a high concentration NO₂ gas.

The volume of the chamber 1 is preferably around 1 to 300 L, morepreferably around 20 to 150 L, and most preferably around 30 to 70 Lsuch that the time required for elevating the concentration of NO₂ ondemand is not too long. The chamber 1 of the present Embodiment has avolume of 40 L.

The chamber 1 is formed by using such as stainless, nickel-chrome alloy,or unsaturated polyester resin (FRP) which is not likely to be corrodedby NO₂ or nitric acid, and the chamber is stably supported by securingit on a base (not shown).

In the present Embodiment, the flow resistive portion 5 is formed by anorifice 5 a. The orifice 5 a is provided in the pipe at the downstreamside from the chamber 1, and makes up an orifice fluid meter. In thepresent Embodiment, therefore, it is advantageous that a flow volume ofthe gas circulating out of the chamber 1 can be measured. In addition tothe orifice 5 a, the flow resistive portion 5 may be configured in sucha manner that a portion of the pipe at the downstream side from thechamber 1 is configured by a narrow pipe to increase the flowresistivity of that portion.

As shown in FIGS. 2 and 3, the plasma generator 2 is a unit capable ofgenerating a plasma under normal temperature and normal pressure byusing microwaves, and is generally configured to include a microwavegenerating apparatus 2 a for generating microwaves with a predeterminedwavelength, a waveguide 2 b which is connected to the microwavegenerating apparatus 2 a to transmit the microwaves, and a plasmagenerating portion 2 c which is provided integrally with the waveguide 2b.

The microwave generating apparatus 2 a generates microwaves at 2.45 GHz,for example, and transmits the microwaves into the waveguide 2 b. Themicrowave generating apparatus 2 a, therefore, includes a microwavegenerating source such as a magnetron for generating microwaves, anamplifier for adjusting the power of the microwaves generated at themicrowave generating source to a predetermined power intensity, and amicrowave transmitting antenna for emitting the microwaves into thewaveguide 2 b. As the microwave generating apparatus 2 a used in theplasma generator 2, an apparatus of a continuous variable type which iscapable of outputting microwave energy of between 1 W and 3 kW, forexample, is suitable.

The waveguide 2 b is made of a nonmagnetic metal (such as aluminum), forexample, has a tubular shape with a rectangular cross section, andtransmits the microwaves generated by the microwave generating apparatus2 a toward the plasma generating portion 2 c. The waveguide 2 b of thepresent Embodiment is configured by a square tubular assembly using topand bottom plates, and two side plates made of metallic flat plates. Inaddition to such plate assembly, the waveguide may also be formed bysuch as extrusion or bending process of a plate member. Moreover, awaveguide 2 b with an oval cross section may be used in addition to thewaveguide 2 b with a rectangular cross section. Furthermore, not only bynonmagnetic metals, but the waveguide may be configured by variousmembers having the waveguide property. The waveguide 2 b is grounded inthe present Embodiment.

The plasma generating portion 2 c is integrally configured with thewaveguide 2 b, and includes a rod-shaped conducting shaft 2 d insertedthrough the waveguide 2 b and a tubular conducting tube 2 e. Theconducting shaft 2 d is further configured by an antenna portion 2 fwhich is inserted into the waveguide 2 b to receive the microwaves, anda center electrode 2 g protruding externally from the waveguide 2 bwhich, in the present Embodiment, is inserted through the waveguide 2 bvia an electric insulator. The conducting shaft 2 d of the presentEmbodiment has a circular cross section, however, the conducting shaftwith an elliptical, oval, or a long oval cross section may be employed.The conducting shaft 2 d of the present Embodiment is formed by usingtitanium, however, materials capable of conducting microwaves such astitanium alloy, copper, platinum, gold, and silver may be used. Ashielding film 2 h made of ceramic is formed at a tip of the conductingshaft 2 d (center electrode 2 g) for preventing arc discharge andprotecting the electrode.

In the present Embodiment, the conducting tube 2 e has a generallycylindrical shape, and the inner diameter thereof is formed to be largerthan the outer diameter of the conducting shaft 2 d. The conducting tubeis provided to cover the center electrode 2 g protruding externally fromthe waveguide 2 b while having the center electrode at the center, and aring-shaped space 2 i is formed between the center electrode 2 g andconducting tube 2 e. The base end of the conducting tube 2 e iselectrically conductive and fixed relative to the waveguide 2 b, and theconducting tube 2 e is thus grounded via waveguide 2 b. The conductingtube 2 e may have such as a rectangular cross section or an oval crosssection in addition to a circular cross section. However, a tip thereofis formed to have a length which terminates with substantially the sameposition as the tip of the center electrode 2 g. It is noted that theconducting tube 2 e of the present Embodiment is made using stainlesssteel, however, it may be made of such as aluminum.

In the conducting tube 2 e of the present Embodiment, a ventilationopening is provided at a position toward the base end thereof. Byconnecting a pipe 2 j extending from the flow resistive portion 5 to theventilation opening, the circulating path 4 connecting from the flowresistive portion 5 to the plasma generator 2 is configured. The gasmixture flowing in the circulating path 4 moves through inside thering-shaped space 2 i toward the end of the center electrode 2 g.Furthermore, to an outside edge of the conducting tube 2 e, a ceramicshielding tube 2 k is inserted to prevent the arc discharge relative tothe center electrode 2 g. The outside edge of the shielding tube 2 k isconnected to the pipe 2 j directing further toward the downstream of thepath to thereby form the circulating path 4.

In the plasma generating portion 2 c thus configured, 2.45 GHz ofmicrowave (power is adjustable) generated from the microwave generatingapparatus 2 a (magnetron) is emitted from the microwave transmittingantenna of the microwave generating apparatus 2 a provided at one end ofthe waveguide 2 b to the plasma generating portion 2 c. The emittedmicrowave transmits in the waveguide 2 b and is received by the antennaportion 2 f of the conducting shaft 2 d in the plasma generating portion2 c. The microwave thus received by the antenna portion 2 f transmits onthe surface of the conducting shaft 2 d, and reaches the tip of thecenter electrode 2 g. The tip of the center electrode 2 g iselectrically coupled to the waveguide 2 b, and is disposed nearby thetip of the conducting tube 2 e of a ground potential. By the microwavesreached the tip of the center electrode 2 g, an intense electric fieldis formed between the tip of the conducting tube 2 e and the tip of thecenter electrode 2 g, especially in the vicinity of the tip of thecenter electrode 2 g. It is noted that the conducting tube 2 e is formedto have a resonance point in the 2.45 GHz band, such that an intenseelectric field is efficiently formed at the tip portion of the centerelectrode 2 g.

By the intense electric field thus formed, partial ionization isgenerated in nitrogen and oxygen included in the gas mixture suppliedvia the circulating path 4. As a result, an aggregate of electrons atseveral tens of thousands degrees in Celsius, ions at substantiallynormal temperature, neutral atoms at substantially normal temperature,and neutral molecules at substantially normal temperature is composed.Comprehensively, this condition is electrically neutral, and in otherwords, a plasma state, and more particularly a low-temperature plasma(non-equilibrium plasma) state is formed.

In other words, nitrogen and oxygen of the gas mixture in the vicinityof the end of the center electrode 2 g generate dielectric breakdown bybeing excited through the intense electric field formed by themicrowaves, and are displaced from the molecular state to thelow-temperature plasma (non-equilibrium plasma) state. The gas under thelow-temperature plasma state has a high reactivity with respect to othergases under the low-temperature plasma state or molecular state.Therefore, when the gas mixture including primarily nitrogen and oxygenis introduced to the plasma generating portion 2 c, a portion thereof isconverted to nitrogen oxides of such as nitrogen monoxide and nitrogendioxide or to ozone.N₂+O₂→2NO  1.N₂+2O₂→2NO₂  2.3O₂→2O₃  3.

It is noted that the conversion ratio is the largest in the case ofequation 1. A portion of nitrogen monoxide generated according toequation 1 binds with oxygen under the low-temperature plasma state inthe plasma generating portion 2 c and is converted to nitrogen dioxide.2NO+O₂→2NO₂  4.

The NOx gas mixture including NO₂ thus generated circulates through thecirculating path 4 or is retained in the chamber 1. During this time,nitrogen monoxide generated according to equation 1 reacts stepwise withoxygen in the NOx gas mixture or with the ozone generated according toequation 3, and is further converted to nitrogen dioxide as shown inequations 5 and 6. As a result, the NO₂ concentration increases.2NO+O₂→2NO₂  5.NO+O₃→NO₂+O₂  6.

Ozone generated according to equation 3 reacts with nitrogen in the NOxgas mixture to generate nitrogen monoxide.N₂+2O₃→2NO+2O₂  7.

This nitrogen monoxide is also converted to nitrogen dioxide by thereactions according to equations 5 and 6.

In this manner, when the NOx gas mixture repeats to circulate in thecirculating path, the concentration of nitrogen dioxide graduallyincreases and the high concentration NO₂ gas with a desired level of NO₂concentration is obtained. However, when the generated nitrogen monoxideor nitrogen dioxide again passes through the plasma generator 2, aphenomenon occurs that a portion thereof again becomes under thelow-temperature plasma state by a dissociation reaction and thus returnsto nitrogen or oxygen. Accordingly, when the concentration of the NOxgas mixture reaches a certain level of the high concentration NO₂ gas byrepeating the circulation, the generation of nitrogen oxide and thedissociation of nitrogen oxide fall under an equilibrium state, so thatthe enhancement does not proceed further at a certain concentration.

In the high concentration NO₂ gas generating system of the presentEmbodiment, a circulating path 4 including a single plasma generator 2is illustrated as shown in FIG. 1. However, two or three or more plasmagenerators 2 may be connected in parallel to form the circulating path4. This is preferable since the high concentration NO₂ gas can begenerated in a short time in such case. Furthermore, the circulatingpath 4 may be divaricated in the plasma generator 2 to provide a plasmagenerating portion 2 c for each of the diverged paths.

The circulating apparatus 3 is configured by using a pressure device 6in the present Embodiment. A fan may also be used as the circulatingapparatus 3. As the pressure device 6, an air pump may be preferablyemployed, and an air blower or air compressor may also be used. As forthe air pump, a diaphragm pump with approximately 20 to 100 watt powerand made of fluorine rubber, a plunger pump made of ceramic, or bellowspump may be employed. The pressure device 6 is provided in the pipe forconnecting the plasma generator 2 and the chamber 1, and is connected toapply pressure to the chamber 1 side at the downstream side of the path.

As mentioned above, the high concentration NO₂ gas generating system ofthe present Embodiment makes up the cyclic circulating path 4 byconnecting the chamber 1, flow resistive portion 5, plasma generator 2,and pressure device 6 in circular via the pipes. By the operation of thepressure device 6, the air (gas mixture) introduced from the inletportion 1 a flows through the circulating path 4, and the NOx gasmixture is generated which includes nitrogen monoxide and nitrogendioxide generated by the reaction of nitrogen and oxygen displaced tothe low-temperature plasma (non-equilibrium plasma) state when passingthrough the plasma generator 2. The nitrogen monoxide is converted tonitrogen dioxide when it reacts with oxygen in the NOx gas mixture andozone stepwise. The high concentration NO₂ gas can thus be generated bythe gradual increase in the concentration of the nitrogen dioxide.

In the high concentration NO₂ gas generating system of the presentEmbodiment, when the NOx gas mixture (including gas mixture) circulatesin the circulating path 4 by the operation of the pressure device 6, thegas pressure increases by the pressure device 6. The gas pressure of theNOx gas mixture gradually decreases in the course of the movement of thegas mixture through the circulating path 4 by the resistance of the flowresistive portion 5 as well as the resistance of each path including theresistance in the pipes, and the gas mixture returns to the pressuredevice 6. As a result, the gas pressures are different in respectiveregions within the path to create a pressure gradient. Particularly, inthe high concentration NO₂ gas generating system, the plasma generator 2is connected to the flow resistive portion 5 at the downstream side ofthe path, and further the pressure device 6 for increasing the pressureat the chamber side is connected downstream thereto. The system isconfigured such that the pressure decreases at the flow resistiveportion 5, and the internal pressure of the plasma generator 2 at thedownstream side of the path is the lowest in the circulating path 4. Thegas pressure of the NOx gas mixture moving nearby the center electrode 2g of the plasma generating portion 2 c is thus maintained at a lowpressure. Accordingly, even if the nitrogen and oxygen in the NOx gasmixture decrease by continuing the gas circulation, the generation ofplasma is stably maintained.

According to the present Embodiment, an NO₂ concentration measuringmeans 7 is further provided in the circulating path 4. The NO₂concentration measuring means 7 employs a sensor which measures aconcentration by projecting blue light on a transparent light guidingtube, which is provided at the chamber 1 and is filled with the NOx gasmixture by communicating therewith, to measure the intensity of thetransmitting light attenuated in accordance with the concentration ofthe NOx gas mixture contained in the light guiding tube by means of alight receiving portion. In addition to this, as the NO₂ concentrationmeasuring means 7, a sensor or the like may be used which uses thesingle-wavelength laser induced fluorescence or which detects NO₂ bycoloring a detecting element utilizing a coupling reaction and measuringthe color intensity of the detecting element.

According to the present Embodiment, the NO₂ concentration measuringmeans 7 is connected to the chamber 1 to measure the NO₂ concentrationof the NOx retained in the chamber 1. As mentioned above, the pressureof the NOx gas mixture flowing through the circulating path 4 generatesdifferences in respective regions. In addition, since the temperature ofthe NOx gas mixture in the plasma generating portion 2 c increases,there is a temperature gradient over the circulating path 4.Accordingly, when the NO₂ concentration is measured, the correction ofthe concentration needs to be performed due to the pressure andtemperature differences at different measurement positions. However, inthe case of measuring the NO₂ concentration of the NOx gas mixtureretained in the chamber 1 containing the high concentration NO₂ gasgenerated according to the present Embodiment, an accurate measurementcan advantageously be performed without needing the correction of theconcentration from the perspective of such pressure and temperature. Itis noted that, in addition to providing in the chamber 1, the NO₂concentration measuring means 7 is preferably provided between thechamber 1 and the flow resistive portion 5 located at the downstream ofthe path from the chamber from the same reason.

Hereinafter, an Embodiment of the method for generating the highconcentration NO₂ gas is described. The method for generating the highconcentration NO₂ gas according to the present Embodiment is performedby using the above-described high concentration NO₂ gas generatingsystem, and is characterized in that the NOx gas mixture is circulatedin the circulating path 4 formed by the chamber 1, flow resistiveportion 5, plasma generator 2, and pressure device 6 until the NO₂concentration reaches 500 ppm to 100,000 ppm. More specifically, themethod includes:

-   -   (1) exhausting (vacuuming) the air in the circulating path 4        including the chamber (exhausting step),    -   (2) filling the dry gas mixture (dry air) in the circulating        path 4 including the chamber (air charging step),    -   (3) starting the plasma generator 2 to generate the NOx gas        mixture including NO₂ from nitrogen and oxygen in the dry air        being displaced to the low-temperature plasma (non-equilibrium        plasma) state (starting step),    -   (4) generating the high concentration NO₂ gas by circulating the        NOx gas mixture until the NO₂ concentration reaches 500 ppm to        100,000 ppm (circulating step), and    -   (5) supplying the high concentration NO₂ gas externally from the        chamber 1 (supplying step).

In the exhausting step, the gas remaining in the circulating path 4including the chamber 1 is released externally by using an exhaust pump1 b to obtain a substantially vacuumed state within the circulating path4. By the step, impurities, moisture and the like remaining in thecirculating path 4 are discharged.

Subsequently in the air charging step, the closure means 1 d of theinlet pipe 1 c is opened to introduce the external fresh air (gasmixture) into the chamber 1. In the present Embodiment, the ambient airin the installation space of the high concentration NO₂ gas generatingsystem is used as the gas mixture. Since the control and operation of agas cylinder filled with the gas mixture is unnecessary, it is excellentin the perspective of workability and cost, and is preferable ingenerating the high concentration NO₂ gas on demand.

At this time, for the purpose of preventing the attachment of impuritiesto such as the plasma generator 2 and of inhibiting nitrification of theNOx gas mixture, the dew point of the gas mixture is dried to be, forexample, from 0 to −90° C., preferably from −30 to −60° C., and −60° C.in the present Embodiment by using the gas drying means 1 e provided inthe air inlet pipe 1 c. In the case the dew point is higher than 0° C.,attachment of impurities to such as the plasma generator 2 is excessivedue to the moisture in the gas mixture, and NO₂ decreases since thenitrification of the NOx gas mixture proceeds. On the other hand, in thecase the dew point is lower than −90° C., time and cost for drying thegas mixture by using the gas drying means 1 e increase. Here, therelationship between the dew point and absolute humidity is described.Since one molecule of H₂O reacts with NO₂ to generate HNO₃, 2.556 mg ofNO₂ is converted to nitric acid by the presence of 1 mg of H₂O. In thecase the dew point is 0° C., since the absolute humidity is 4.46 mg(mg/L), 11.39 (mg/L) of NO₂ is converted to nitric acid. On the otherhand, in the case the dew point is −30° C., since the absolute humidityis 0.28 mg (mg/L), 0.71 (mg/L) of NO₂ is converted to nitric acid suchthat the effect of moisture can be made to be 1% or less. In the samemanner, 0.24 (mg/L) of NO₂ is converted to nitric acid in the case thedew point is −40° C., and 0.00018 (mg/L) of NO₂ is converted to nitricacid in the case the dew point is −90° C. In other words, the lower thedew point is, the lower the amount of nitric acid to be converted,resulting in the effective use of the generated high concentration NO₂gas. However, as mentioned above, from the perspective of the increasingtime and cost for drying by using the gas drying means 1 e, the dewpoint is preferably between −30 and −60° C.

In the air charging step, by introducing the air into the circulatingpath 4 under a substantially vacuumed state, the pressure in thecirculating path 4 including the inside of the chamber 1 increases. Atthe time when the differential pressure between the increasing internalpressure and the external pressure is between −1 and −50 kPa (relativepressure), the air drive valve provided as the closure means 1 d isclosed to stop the air supply. In this manner, a “negative pressurestate prior to start up” in which the internal pressure of thecirculating path 4 is lower than the external pressure is created.

Subsequently in the circulating step, the microwave generating apparatus2 a of the plasma generator 2 and the pressure device 6 are started. Byway of this, the gas mixture circulates in the circulating path 4, andnitrogen oxide of such as nitrogen monoxide and nitrogen dioxide, andozone are generated to create the NOx gas mixture by the displacement ofnitrogen and oxygen of the gas mixture into the low-temperature plasmastate at the plasma generating portion 2 c of the plasma generator 2. Byfurther circulating the NOx gas mixture, the NO₂ concentration isgradually increased in the above-described manner. The circulation ofthe NOx gas mixture is continued until the concentration of NO₂ reaches,for example, approximately 500 to 100,000 ppm, preferably 20,000 to60,000 ppm, and 40,000 ppm in the present Embodiment to generate thehigh concentration NO₂ gas. In the case the NO₂ concentration of thehigh concentration NO₂ gas is less than 500 ppm, the sterilizationeffect may not be sufficient for a microspace of an item to besterilized such as an inside of a tube. On the other hand, in the casethe concentration is above 100,000 ppm, the sterilization effect is notfurther increased and the exhausting process of the high concentrationNO₂ gas becomes troublesome, and the time and cost for generating thehigh concentration NO₂ gas substantially increase.

When the pressure device 6 is actuated and the gas mixture or the NOxgas mixture circulates in the circulating path 4, the internal pressureof the path creates the pressure gradient in which the pressure is thehighest at the downstream side of the path from the pressure device 6and gradually decreases toward the downstream of the path due to theresistance in the flow resistive portion 5, the resistance in the plasmagenerating portion 2 c, the resistance in the pipes and the like. Sincethe circulation is initiated from the above-described negative pressurestate prior to start up, the pressure gradient is created by setting thenegative pressure prior to start up as the mean value. Furthermore, inthe present Embodiment, the negative pressure prior to start up is setby adjusting the timing for operating the closure means 1 d such thatthe pressure at the downstream from the pressure device 6 with thehighest internal pressure is still lower than the atmospheric pressure.More specifically, the pressure difference between the atmosphericpressure and the gas mixture or the NOx gas mixture present in theinterval from the pressure device 6 through the chamber 1 connected tothe pressure device 6 at the downstream side therefrom to the flowresistive portion 5 connected to the chamber 1 at the downstream sidetherefrom is set, for example, between approximately −1 and −50 kPa(relative pressure), preferably −5 and −40 kPa (relative pressure), and−5 kPa (relative pressure) in the present Embodiment. In the case thedifferential pressure is less than −1 kPa (relative pressure), this maylead to leakage of the NOx gas through the connecting portions of thecirculating path, air inlet portion 1 a for introducing the gas mixture,or the gas supply opening for taking out the high concentration NO₂. Onthe other hand, in the case the differential pressure is above −50 kPa(relative pressure), it is excessive for the purpose of preventing thegas leakage, and the amount of NO₂ in the high concentration NO₂ gas islikely to decrease. Here, the internal pressure of the chamber 1 and thevolume allowed for storing the high concentration NO₂ gas are described.In the case the volume of the chamber 1 is 40 L and the atmosphericpressure is 101.3 kPa (absolute pressure), the volume allowed forstoring the high concentration NO₂ gas is 40 L when the internalpressure of the chamber 1 is 0 kPa (relative pressure). The volumeallowed for storing the high concentration NO₂ gas is larger when theinternal pressure is positive. However, this is not preferable since thehigh concentration NO₂ gas stored in the chamber may leak as describedabove. On the other hand, the volume allowed for storing the highconcentration NO₂ gas is 36.1 L when the internal pressure of thechamber 1 is −10 kPa (relative pressure). The volume allowed for storingthe high concentration NO₂ gas is 28.2 L when the internal pressure ofthe chamber 1 is −30 kPa (relative pressure), and 20.3 L when thepressure is −50 kPa (relative pressure). In other words, the lower theinternal pressure in the chamber 1, the smaller the volume allowed forstoring the high concentration NO₂ gas. Therefore, the internal pressureof the chamber 1 is preferably negative, and the pressure from −1 to −50kPa (relative pressure) is more preferable. The lower limit is −50 kPa(relative pressure) from the perspective of the decreasing volumeallowed for storing the high concentration NO₂ gas, however, there is noproblem from the safety perspective when the pressure is lower than thatvalue. With respect to the internal pressure of the circulating path 4,provided that leakage of gas is prevented by a suitable leakageprevention means, the internal pressure of the interval from thepressure device 6 through the chamber 1 to the flow resistive portion 5may set to be a positive pressure relative to the atmospheric pressureby delaying the timing for operating the closure means 1 d. Under a highpressure, the amount of NO₂ in the high concentration NO₂ gas increasesat the same level of concentration. This is preferable in that a largeamount of NO₂ can be generated while using a small chamber 1.

By adjusting the timing for operating the closure means 1 d or adjustingthe value of resistance of the flow resistive portion 5, the pressureinside the plasma generating portion 2 c of the plasma generator 2 ispreferably set between approximately 20 and 90 kPa (absolute pressure),more preferably 40 and 80 kPa (absolute pressure), and 70 kPa (absolutepressure) in the present Embodiment. The dielectric breakdown due to themicrowaves for generating a plasma is intensified with a lower gaspressure. Under the small negative pressure of above 90 kPa (absolutepressure), the stability for generating a plasma due to the dielectricbreakdown decreases. Particularly, at the stage when the circulation ofthe NOx gas mixture proceeds and the nitrogen and oxygen contents aredecreased, the generation of plasma may stop. On the other hand, in thecase of a low pressure of less than 20 kPa (absolute pressure), thedielectric breakdown is accelerated. However, the amount of oxygen andnitrogen in the gas mixture or NOx gas mixture is likely to decrease.

The flow volume of the NOx gas mixture circulating in the circulatingpath 4 is preferably 5 LPM or more. With a small flow volume of lessthan 5 LPM, the gas flow rate flowing in the ring-shaped space 2 ibetween the center electrode 2 g of the plasma generator 2 and theconducting tube 2 e decreases. This may lead to damage the centerelectrode 2 g in a short time period, since the cooling effect for thewarming center electrode 2 g is not sufficient. On the other hand, thereis no significant problem associated with an increased flow volume.However, this places undue load on the pressure device 6 and increasesthe operating costs. In view of this, it is preferable when the upperlimit is set to be approximately 200 LPM.

In the circulating step, when the concentration of NO₂ increases, and atthe time when the high concentration NO₂ gas (NO₂ concentration of40,000 ppm in the present Embodiment) is reached as a result of themeasurement taken by the NO₂ concentration measuring means 7, theoperation of the plasma generator 2 and pressure device 6 is stopped. Inthe subsequent supplying step, the high concentration NO₂ gas filled inthe chamber 1 is supplied from the gas supply opening to a sterilizingchamber 1 for containing such as medical instruments for sterilizationin the present Embodiment. It is noted that the gas is supplied throughthe suction of the vacuumed sterilizing chamber 1 in the presentEmbodiment, however the gas may be supplied by using a pump.

EXAMPLE

Hereinafter, the high concentration NO₂ generating system of the presentinvention is described in detail by Examples. However, the presentinvention is not limited to those Examples.

Generation Speed of NO₂ in the Case of Changing the Number of Electrodes(Conducting Shaft 2 d) Example 1

In the circulating path shown in FIG. 4, when the internal pressure ofthe chamber 1 was vacuum (−101 kPa (relative pressure)), the air (dewpoint −60° C.) was filled such that the internal pressure of the chamber1 was to be −5 kPa (relative pressure). The air was made to circulate bya pressure device 61 and pressure device 62, and a flow resistiveportion 51 and flow resistive portion 52 were adjusted such that theflow meter F1 and flow meter F2 indicated 16 LPM. The pressure in thecirculating path at this time was monitored to be from 60 to 70 kPa(absolute pressure) with a pressure meter PG1 and pressure meter PG2.With a plasma generator 21 and plasma generator 22, two electrodes wereinserted in the waveguide 2 b to apply 160 W of electric power to theplasma generator 21 and plasma generator 22. The concentration of thehigh concentration NO₂ gas was measured over time by the NO₂concentration measuring means 7. The result is shown in FIG. 5. Thereference numeral E1 is a graph showing the changes of the concentrationof the high concentration NO₂ gas over time according to Example 1.

Example 2

Two electrodes were inserted in the plasma generator 21 to apply 160 Wof electric power. Electric power was not applied to the other plasmagenerator 22. Other arrangement was the same as that of Example 1, andthe concentration of the high concentration NO₂ gas was measured overtime. The result is shown in FIG. 5. The reference numeral E2 is a graphshowing the changes of the concentration of the high concentration NO₂gas over time according to Example 2.

Example 3

One electrode was inserted in the plasma generator 21 to apply 80 W ofelectric power. Electric power was not applied to the other plasmagenerator 22. Other arrangement was the same as that of Example 1, andthe concentration of the high concentration NO₂ gas was measured overtime. The result is shown in FIG. 5. The reference numeral E3 is a graphshowing the changes of the concentration of the high concentration NO₂gas over time according to Example 3.

From Examples 1 to 3, it was found that the increase in theconcentration of the high concentration NO₂ is faster when the number ofthe electrodes (conducting shaft 2 d) is increased. The concentrationreached 70 mg/L in approximately 60 minutes in Example 1, theconcentration reached 70 mg/L in approximately 120 minutes in Example 2,and the concentration reached 70 mg/L in approximately 240 minutes inExample 3. Therefore, it was found that the number of electrodes(conducting shaft 2 d) is proportional to the generating speed of thehigh concentration NO₂ gas.

Here, the concentration of NO₂ gas generated in Examples 1 to 3 wasapproximately 70 mg/L, and it is 36,500 ppm when the unit is convertedto ppm. In addition, since NO₂ and N₂O₄ are present under an equilibriumstate in the high concentration NO₂ gas, 63,600 ppm of NO₂ istheoretically present in practical.

(Generation Speed of NO₂ in the Case of Changing the Flow Volume of NOxGas Mixture Circulating in the Circulating Path)

Example 4

In the circulating path shown in FIG. 6, when the internal pressure ofthe chamber 1 was vacuum (−101 kPa (relative pressure)), the air (dewpoint −60° C.) was filled such that the internal pressure of the chamber1 was to be −5 kPa (relative pressure). The air was made to circulate bythe pressure device 61 and pressure device 62, and the flow resistiveportion 51, flow resistive portion 52, and flow resistive portion 53were adjusted such that the internal pressure of the plasma generator 2was to be from 60 to 70 kPa (absolute pressure), and the flow volume ofthe gas was to be 8 LPM±1 LPM. Two electrodes were respectively insertedto the plasma generator 21 and plasma generator 22, and electric powerapplied to the plasma generator 21 and plasma generator 22 was 160 W,respectively. The concentration of the high concentration NO₂ gas wasmeasured over time by the NO₂ concentration measuring means 7. Theresult is shown in FIG. 7. The reference numeral E4 is a graph showingthe changes of the concentration of the high concentration NO₂ gas perelectrode over time according to Example 4.

Example 5

Other than that the flow resistive portion 51, flow resistive portion52, and flow resistive portion 53 were adjusted to obtain the flowvolume of the gas of 5 LPM±1 LPM, the arrangement was the same as thatof Example 4. The concentration of the high concentration NO₂ gas wasmeasured over time. The result is shown in FIG. 7. The reference numeralE5 is a graph showing the changes of the concentration of the highconcentration NO₂ gas per electrode over time according to Example 5.

Example 6

Other than that the flow resistive portion 51, flow resistive portion52, and flow resistive portion 53 were adjusted to obtain the flowvolume of the gas of 12 LPM±1 LPM, and a pressure device 63 and pressuredevice 64 were used together, the arrangement was the same as that ofExample 4. The concentration of the high concentration NO₂ gas wasmeasured over time. The result is shown in FIG. 7. The reference numeralE6 is a graph showing the changes of the concentration of the highconcentration NO₂ gas per electrode over time according to Example 6.

From Examples 4 to 6, it was found that the generation speed of the highconcentration NO₂ gas is the same level in the case the flow volume ofthe gas is 8 LPM or above, and the difference was little even when theflow volume is 5 LPM.

(The Concentration of High Concentration NO₂ Gas in the Case NOx GasMixture is Not Circulated in the Circulating Path)

Comparative Example 1

In the circulating path shown in FIG. 8, when the internal pressure ofthe chamber 1 was vacuum (−101 kPa (relative pressure)), the air (dewpoint −60° C.) was filled such that the internal pressure of the chamber1 was to be −5 kPa (relative pressure). The pressure device 61 andpressure device 62 were actuated, and the flow resistive portion 53 wasreleased. The flow resistive portion 51, flow resistive portion 52, flowresistive portion 54, and the flow meter F1 were adjusted such that theinternal pressures of the plasma generators 21 and 22 were to be from 60to 70 kPa (absolute pressure), the flow volume was to be 16 LPM±1 LPM (8LPM per electrode), and the internal pressure of the chamber 1 was to be−5 kPa (relative pressure). The pressure device 61 and pressure device62 were stopped, and the flow resistive portion 53 was closed such thata middle chamber MC was made to be vacuumed (−95 kPa (relativepressure)). After adjusting the flow volume in the plasma generator 2 tobe 5 LPM by actuating the pressure device 61 and pressure device 62, theplasma was ignited. The concentration of the high concentration NO₂ gaswas measured by an NO₂ concentration measuring means 71 and NO₂concentration measuring means 72. The number of electrodes was four, and160 W of electric power was applied. The result is shown in FIG. 9. Thereference numeral CE1 a is a graph showing the changes of theconcentration of the high concentration NO₂ gas per electrode over timeimmediately after the plasma according to Comparative Example 1. Thereference numeral CE1 b is a graph showing the changes of theconcentration of the high concentration NO₂ gas per electrode over timeimmediately after the middle chamber MC according to ComparativeExample 1. The reference numeral T is a dry air storage tank, and thereference numeral F3 is an automatic flow volume adjusting mechanism.

Comparative Example 2

Other than that after vacuuming the middle chamber MC, the flow volumein the plasma generator 2 was adjusted to be 8 LPM by actuating thepressure device 61 and pressure device 62, the arrangement was the sameas that of Comparative Example 1. The concentration of the highconcentration NO₂ gas was measured. The result is shown in FIG. 9. Thereference numeral CE2 a is a graph showing the changes of theconcentration of the high concentration NO₂ gas per electrode over timeimmediately after the plasma according to Comparative Example 2. Thereference numeral CE2 b is a graph showing the changes of theconcentration of the high concentration NO₂ gas per electrode over timeimmediately after the middle chamber MC according to Comparative Example2.

From FIG. 9, it was found that the concentration of NO₂ does notincrease and is at most 7 mg/L (36.50 ppm when the unit is converted toppm), and it was found the NO₂ gas can be enhanced by circulation.

Lightning Stability of Plasma by Internal Pressure of Plasma GeneratingPortion Example 7

In the circulating path shown in FIG. 10, when the internal pressure ofthe chamber 1 was vacuum (−101 kPa (relative pressure)), the air (dewpoint −60° C.) was filled such that the internal pressure of the chamber1 was to be −5 kPa (relative pressure). The internal pressure of theplasma generator 2 was made to be −20 kPa (relative pressure) byactuating the pressure device 6, and the flow resistive portion 51, flowresistive portion 53, and flow meter F1 were adjusted such that the flowvolume in the plasma generator 2 was to be 16 LPM±1 LPM (8 LPM perelectrode). The number of electrode was two, and 120 of electric powerwas applied. The plasma lightning time was 1 hour. The test wasperformed three times, and an average was calculated. The result isshown in Table 1.

Comparative Example 3

In the circulating path shown in FIG. 11, when the internal pressure ofthe chamber 1 was vacuum (−101 kPa (relative pressure)), the air (dewpoint −60° C.) was filled such that the internal pressure of the chamber1 was to be −5 kPa (relative pressure). The internal pressure of theplasma generator 2 was made to be 0 kPa (relative pressure) by actuatingthe pressure device 6, and the flow resistive portion 51, flow resistiveportion 52, and flow meter F1 were adjusted such that the flow volume inthe plasma generator 2 was to be 16 LPM±1 LPM (8 LPM per electrode). Thenumber of electrode was four, and 160 W of electric power was applied.The plasma lightning time was 1 hour. The test was performed threetimes, and an average was calculated. The result is shown in Table 1.

Comparative Example 4

In the circulating path shown in FIG. 11, when the internal pressure ofthe chamber 1 was vacuum (−101 kPa (relative pressure)), the air (dewpoint −60° C.) was filled such that the internal pressure of the chamber1 was to be −5 kPa (relative pressure). The internal pressure of theplasma generator 2 was made to be −20 kPa (relative pressure) byactuating the pressure device 6, and the flow resistive portion 51, flowresistive portion 52, and flow meter F1 were adjusted such that the flowvolume in the plasma generator 2 was to be 16 LPM±1 LPM (8 LPM perelectrode). The number of electrode was four, and 160 W of electricpower was applied. The plasma lightning time was 1 hour. The test wasperformed three times, and an average was calculated. The result isshown in Table 1.

TABLE 1 Internal pressure of Concentration of high concentration plasmagenerating NO₂ gas [mg/L] portion 2 [kPa] Test 1 Test 2 Test 3 AverageExample 7 −20 70 70 70 70 Comparative 0 28 52.7 40.1 40.3 Example 3Comparative 20 13.3 47.1 20.9 27.1 Example 4

As shown in Table 1, the concentration of the high concentration NO₂ gasincreased to 70 mg/L, and the plasma did not turn off in Example 7. Onthe other hand, the plasma turned off during the plasma lightening time(1 hour) in Comparative Example 3 and Comparative Example 4, and theconcentration of the high concentration NO₂ gas increased only to thevalues as shown in Table 1. Accordingly, it was found that thelightening of plasma is maintained in the case the internal pressure ofthe plasma generator 2 is negative, however, plasma is likely to turnoff in the case the internal pressure of the plasma generator 2 is 0 orpositive.

Damage on Electrode in the Case of Changing the Flow Volume of GasComparative Example 5

In the circulating path shown in FIG. 4, when the internal pressure ofthe chamber 1 was vacuum (−101 kPa (relative pressure)), the air (dewpoint −60° C.) was filled such that the internal pressure of the chamber1 was to be −5 kPa (relative pressure). The pressure device 61 andpressure device 62 were actuated, and the flow resistive portion 51 andflow resistive portion 52 were adjusted to control the flow volume ofthe gas such that the flow meter F1 and flow meter F2 indicated 1 LPMfor generating the high concentration NO₂ gas. The pressure was adjustedsuch that the value of the pressure meter PG1 and pressure meter PG2indicated from 60 to 70 kPa (absolute pressure). The number ofelectrodes was four, and 160 W of electric power was applied. The plasmalightening time was 1 hour. As a result, two electrodes out of four weredamaged.

Comparative Example 6

Other than adjusting the flow meter F1, flow meter F2, flow resistiveportion 51, and flow resistive portion 52 such that the flow volume ofthe gas was to be 2 LPM, the high concentration NO₂ gas was generated inthe same manner as in Comparative Example 5. As a result, one electrodeout of four was damaged.

From the result of Comparative Example 5 and Comparative Example 6, itwas found that the electrode body is not sufficiently cooled by a gasflow in the case a gas flow is 5 LPM or less, and the alumina coatprovided on the surface of the electrode made of titanium is damaged asa result.

(Concentration of High Concentration NO₂ Gas in the Case of ChangingRatio of Oxygen in Ingredient Gas)

Example 8

In the circulating path shown in FIG. 10, when the internal pressure ofthe chamber 1 was vacuum (−101 kPa (relative pressure)), an ingredientgas (10% oxygen, 90% nitrogen) was filled such that the internalpressure of the chamber 1 was to be −5 kPa (relative pressure). The flowvolume of the gas was adjusted to be 16 LPM by actuating the circulatingpressure device 6 to generate the high concentration NO₂ gas. Theconcentration thereof was measured by the NO₂ concentration measurementmeans 7. The pressure was adjusted such that the value of the pressuremeter PG1 indicated from 60 to 70 kPa (absolute pressure). The number ofelectrodes was two, and 120 W of electric power was applied. The resultis shown in FIG. 12. The reference numeral E8 is a graph showing theconcentration of the high concentration NO₂ gas over time according toExample 8.

Example 9

Other than that an ingredient gas (20% oxygen, 80% nitrogen) was used,the high concentration NO₂ gas was generated in the same manner as inExample 8. The result is shown in FIG. 12. The reference numeral E9 is agraph showing the concentration of the high concentration NO₂ gas overtime according to Example 9.

Example 10

Other than that an ingredient gas (40% oxygen, 60% nitrogen) was used,the high concentration NO₂ gas was generated in the same manner as inExample 8. The result is shown in FIG. 12. The reference numeral E10 isa graph showing the concentration of the high concentration NO₂ gas overtime according to Example 10.

Example 11

Other than that an ingredient gas (50% oxygen, 50% nitrogen) was used,the high concentration NO₂ gas was generated in the same manner as inExample 8. The result is shown in FIG. 12. The reference numeral E11 isa graph showing the concentration of the high concentration NO₂ gas overtime according to Example 11.

Example 12

Other than that an ingredient gas (60% oxygen, 40% nitrogen) was used,the high concentration NO₂ gas was generated in the same manner as inExample 8. The result is shown in FIG. 12. The reference numeral E12 isa graph showing the concentration of the high concentration NO₂ gas overtime according to Example 12.

Example 13

Other than that an ingredient gas (80% oxygen, 20% nitrogen) was used,the high concentration NO₂ gas was generated in the same manner as inExample 8. The result is shown in FIG. 12. The reference numeral E13 isa graph showing the concentration of the high concentration NO₂ gas overtime according to Example 13.

As shown in FIG. 12, the concentration of the high concentration NO₂ gasincreased to approximately 35 (mg/L) in the case of using the ingredientgas (oxygen 10%, nitrogen 90%) of Example 8, the concentration of thehigh concentration NO₂ gas increased to approximately 62 (mg/L) in thecase of using the ingredient gas (oxygen 20%, nitrogen 80%) of Example9, and the concentration of the high concentration NO₂ gas increased tothe maximum of approximately 80 (mg/L) in the case of using theingredient gas (oxygen 80%, nitrogen 20%) of Example 13. Theconcentration of the high concentration NO₂ gas increased above themaximum of approximately 86 (mg/L) in the case of using the ingredientgas (oxygen 40%, nitrogen 60%) of Example 10, the ingredient gas (oxygen50%, nitrogen 50%) of Example 11, and the ingredient gas (oxygen 60%,nitrogen 40%) of Example 12. In addition, it was found that theincreasing speed is also fast.

The concentration of the high concentration NO₂ gas generated in Example12 is approximately 86 mg/L, and it is 44,900 ppm when the unit isconverted to ppm. In addition, since NO₂ and N₂O₄ are present under anequilibrium state in the high concentration NO₂ gas, 85,800 ppm of NO₂is theoretically present in practical. To further increase theconcentration of NO₂, it is achievable by increasing the electric powerof the plasma generator 2. According to the present invention, theconcentration of NO₂ can therefore be increased to approximately 100,000ppm.

Example 14

Other than that 160 W of electric power was applied to the plasmagenerator and the plasma lightning time was extended, the highconcentration NO₂ gas was generated in the same manner as in Example 11.The result is shown in FIG. 13. The reference numeral E14 is a graphshowing the concentration of the high concentration NO₂ gas over timeaccording to Example 14. As shown in FIG. 13, the total amount of NO₂and N₂O₄ present under an equilibrium state was approximately 100,000ppm.

INDUSTRIAL APPLICABILITY

According to the high concentration NO₂ generating system of the presentinvention and the method for generating high concentration NO₂ using thegenerating system, NO₂ can be simply and selectively concentrated(approximately 500 ppm or above). In addition, since a gas mixtureincluding nitrogen and oxygen is used as an ingredient, the managementof ingredients is simple and highly safe, and the high concentration ofNO₂ can be simply and selectively prepared on demand.

EXPLANATION OF SYMBOLS

-   1 chamber-   1 a inlet portion-   1 b exhaust pump-   1 c inlet pipe-   1 d, 1 g, V1 closure means-   1 e gas drying means-   1 f exhaust pipe-   1 h exhaust pump-   2 plasma generator-   2 a microwave generating apparatus-   2 b waveguide-   2 c plasma generating portion-   2 d conducting shaft-   2 e conducting tube-   2 f antenna portion-   2 g center electrode-   2 h shielding film-   2 i ring-shaped space-   2 j pipe-   2 k shielding tube-   3 circulating apparatus-   4 circulating path-   5 flow resistive portion-   5 a orifice-   6 pressure device-   7 NO₂ concentration measurement sensor-   8 pressure detecting means-   CE1 a concentration of the high concentration NO₂ gas per electrode    over time immediately after plasma according to Comparative Example    1-   CE1 b concentration of the high concentration NO₂ gas per electrode    over time immediately after middle chamber according to Comparative    Example 1-   CE2 a concentration of the high concentration NO₂ gas per electrode    over time immediately after plasma according to Comparative Example    2-   CE2 b concentration of the high concentration NO₂ gas per electrode    over time immediately after middle chamber according to Comparative    Example 2-   E1 concentration of high concentration NO₂ gas according to Example    1

E2 concentration of high concentration NO₂ gas according to Example 2

-   E3 concentration of high concentration NO₂ gas according to Example    3-   E4 concentration of high concentration NO₂ gas per electrode    according to Example 4-   E5 concentration of high concentration NO₂ gas per electrode    according to Example 5-   E6 concentration of high concentration NO₂ gas per electrode    according to Example 6-   E8 concentration of high concentration NO₂ gas according to Example    8-   E9 concentration of high concentration NO₂ gas according to Example    9-   E10 concentration of high concentration NO₂ gas according to Example    10-   E11 concentration of high concentration NO₂ gas according to Example    11-   E12 concentration of high concentration NO₂ gas according to Example    12-   E13 concentration of high concentration NO₂ gas according to Example    13-   F1, F2 flow meter-   F3 flow volume adjusting mechanism-   f filter-   T dry air storage tank-   MC middle chamber-   PG1, PG2 pressure device

What is claimed is:
 1. A method for generating high concentration NO₂,comprising circulating a NOx gas mixture in a circulating path formed bya chamber, a plasma generator, and a circulating means until NO₂concentration reaches 500 ppm to 100,000 ppm by using a highconcentration NO₂ gas generating system comprising: the circulating pathconfigured by connecting the chamber, the plasma generator, and thecirculating means; wherein NO₂ is generated by circulating the NOx gasmixture including nitrogen and oxygen in the circulating path.
 2. Themethod for generating high concentration NO₂ according to claim 1,wherein ambient air is employed for the gas mixture.
 3. The method forgenerating high concentration NO₂ according to claim 1, wherein dry airwith a dew point from 0 to −90° C. is used for the gas mixture.
 4. Themethod for generating high concentration NO₂ according to claim 1,wherein an internal pressure of a plasma generating portion of saidplasma generator is from 20 to 90 kPa (absolute pressure).
 5. The methodfor generating high concentration NO₂ according to claim 1, wherein apressure difference between atmospheric pressure and an internalpressure of an interval from a pressure device through said chamberconnected to said pressure device at a downstream side of a path to aflow resistive portion connected to said chamber at said downstream sideof said path is set between approximately −1 and −50 kPa (relativepressure).
 6. The method for generating high concentration NO₂ accordingto claim 1, wherein an internal pressure of an interval from a pressuredevice through said chamber connected to said pressure device at adownstream side of a path to a flow resistive portion connected to saidchamber at said downstream side of said path is maintained to be apositive pressure relative to atmospheric pressure.
 7. The method forgenerating high concentration NO₂ according to claim 1, wherein a flowvolume of said NOx gas mixture circulating in said circulating path is 5LPM or above.